In the chemical step, a low concentration of ferric sulfate is used as the lixiviating agent. In the biological step, bacterial films of Thiobacillus ferrooxidans attached to an inert solid are used to regenerate the lixiviating agent by converting the ferrous ion into ferric ion through oxidation. The regenerated agent is then recycled to the lixiviation reactor. The material to be treated by the biolixiviation process contains copper sulfides and includes run-of-mine coal minerals, refined concentrates, aggregates, semi-aggregates and distinctive concentrates. The biolixiviation process permits complete extraction of the copper contained in the ore and results in a lixiviation liquor which contains all the copper charge and a low concentration of ferric sulfate similar to the low concentration of ferric sulfate used initially. The copper obtained can be treated without difficulty by means of extraction with solvents and electrolysis in order to obtain cathode copper.
The treatment of flotation concentrates of copper sulfides has numerous legal and economic disadvantages including increasing legal restrictions in the matter of contamination, the exhaustive saturation of the market with sulfuric acid, tough penalties for the sale of flotation concentrates, the high energy costs of traditional pyrometallurgical treatment processes, and the limited flexibility of such processes with respect to the composition of the raw material. Those reasons, among others, have given rise during the last few decades to a growing interest in hydrometallurgy as an alternative treatment of the flotation concentrates of copper sulfides. The main difficulty with hydrometallurgy is the selection of a suitable method of lixiviation which is economical, effective and flexible. Biolixiviation may be considered a suitable alternative treatment of the flotation concentrates of copper sulfides.
Biolixiviation can be defined as a hydrometallurgical operation, in which various components of a metallic ore are degraded by the action of certain chemolithotrophic microorganisms which obtain the energy necessary for their growth from intracellular oxidation of inorganic substances. The species Thiobacillus ferrooxidans is noteworthy because it uses ferrous ion in solution and any reduced form of sulfur including the metallic sulfides as energy substrates.
The principal disadvantage of biolixiviation of sulfurated metallic ores is the slow kinetics of this type of process which typically has very long reaction times. The slow reaction rate determines to a considerable extent the industrial device to be used for biolixiviation and also the type of mineral which can be treated with this process. Currently, biolixiviation is only used to process low-grade or even marginal-grade minerals and refractory minerals, and the lixiviation process is performed by percolation in heap or dump leaching systems.
The biolixiviation of sulfurated ores can take place through two different mechanisms, known as the indirect contact and direct contact mechanisms. In the indirect contact mechanism, the ferric ion oxidizes the metallic sulfide producing ferrous sulfate and elemental sulfur which are then reoxidized by the bacteria to regenerate the ferric ion with the production of acid. In the direct contact mechanism, the bacterial action is independent of the presence of ferric ion, and only an intimate physical contact between the bacteria and the surface of the mineral is required. The indirect and direct mechanisms have very different reaction kinetics.
When the bacteria uses the metallic sulfide directly as an energy substrate in the direct contact mechanism, its mean reaction time is on the order of days or even weeks, while the reaction time when ferric ion is used in the indirect contact mechanism ranges from 4 to 8 hours. In spite of the difference in reaction times, the reaction kinetics of indirect biolixiviation are considered slow and prevent its use in the treatment of high-grade minerals or flotation concentrates.
The possibility of improved reaction kinetics in the biolixiviation processes of direct contact is based on the establishment of more active species either by the discovery of new naturally occurring species or by the modification of species already used by means of adaptation techniques or even genetic manipulation. However, the implantation of these cultures may run into serious difficulties, especially that of preventing said prepared strains from losing their identity by mutation after a short operating time.
The possibility of improved reaction kinetics of the biolixiviation mechanism of indirect contact are much greater in the short term than for the direct contact mechanism. To improve the reaction rate of the indirect mechanism, it is only necessary to separate the two processes which take place simultaneously in the lixiviation reactor, namely the chemical attack on the metallic sulfide by the ferric ion and the bacterial oxidation of the ferrous ion produced. This separation permits the improvement of the reaction kinetics of both processes.
The attack of the ferric ion on the metallic sulfides is an electrochemical reaction based on the semiconductor properties of these materials. The kinetics of these reactions are greatly affected by the temperature. At ambient temperatures, the reactions are very slow; however, at moderately high temperatures, even temperatures below the boiling point of water, the reactions are considerably faster. The reactions at higher temperatures are faster because, unlike normal conductors, an increase in the temperature increases the conductivity of the semiconductors.
The mesophilic character of the bacteria normally used prevents the use of higher temperatures to accelerate the lixiviation process; however, the physical separation of the chemical and biological steps has the same accelerating effect on the lixiviation process that increased temperature would have caused.
During the chemical reaction stage, the ferric ion, which acts as a depolarizer, is depleted by reduction to the ferrous ion. To prevent the reaction from stopping when the ferric ion reactant is used up, it is necessary to oxidize the ferrous ion formed to regenerate the primary lixiviating agent. At this stage of the lixiviation process, bacteria play a fundamental role by catalyzing said oxidation process to regenerate the ferric ion lixiviating agent.
When the biolixiviation is carried out in a simple stage in a single reactor, a series of phenomena occur which considerably limit the speed of this oxidation process. When a single reactor is used in which the biolixiviation reactions are mixed, significant abrasion of the bacteria by the mineral particles occurs. Abrasion of the bacteria leads to their partial destruction which results in two negative effects on the kinetics of the process. First, at the half-way point of the process, organic substances from the rupture of the cellular membranes cause disintegration of the bacterial cytoplasm which results in pronounced inhibition of bacterial growth. Secondly, the abrasive effect results in a decrease in the active bacterial count.
After several days of operation of the biolixiviating process in a single reactor, the negative effects of abrasion on the bacteria may result in as much as a twenty-fold reduction in the oxidative activity of the bacterial suspension when compared to the oxidative activity of the original inoculation.
Another consideration in the biolixiviation process involves the supply of gases to the bacteria. Bacteria, in addition to being autotrophs, are aerobes which means that they require an adequate supply of carbon dioxide (CO.sub.2) and oxygen (O.sub.2). Carbon dioxide is an exclusive source of carbon for the synthesis of the cellular material and the oxygen is a final acceptor of the pairs of electrons generated in the oxidation processes in which they take part.
It is necessary, then, to use a device that permits a continuous supply of these two gases, in amounts as high as permitted by the saturation conditions of the liquid medium in which they are dissolved.